Leetcode 75 Sort Colors

Given an array with n objects colored red, white or blue, sort them so that objects of the same color are adjacent, with the colors in the order red, white and blue.

Here, we will use the integers 0, 1, and 2 to represent the color red, white, and blue respectively.

Note:
You are not suppose to use the library's sort function for this problem.

为0的前端插入，为2的后端插入，即可完成不稳定的排序。

注意下标的变化。

这里用Ruby的pop()不太好写。

def sortColors(self, A):
    i = 0
    for j in range(len(A)):
        if A[i] == 0:
            A.insert(0, A.pop(i))
        elif A[i] == 2:
            A.append(A.pop(i))
            i -= 1
        i += 1